On the role of precursor powder composition in controlling microstructure, flux pinning, and the critical current density of Ag/Bi 2 Sr 2 CaCu 2 O x conductors

Precursor powder composition is known to strongly affect the critical current density ( J c ) of Ag/Bi 2 Sr 2 CaCu 2 O x (Bi-2212) wires. However, reasons for such J c dependence have not yet been fully understood, compromising our ability to achieve further optimization. We systematically examined superconducting properties, microstructural evolution and phase transformation, and grain boundaries of Bi-2212 conductors fabricated from precursor powders with a range of compositions using a combination of transport-current measurements, a quench technique to freeze microstructures at high temperatures during heat treatment, and scanning transmission electron microscopy (STEM). Samples include both dip-coated tapes and round wires, among which a commercial round wire carries a high J c of 7600 A mm −2 at 4.2 K, self-field and 2600 A mm −2 at 4.2 K, 20 T, respectively. In the melt, this high- J c conductor, made using a composition of Bi 2.17 Sr 1.94 Ca 0.89 Cu 2 O x , contains a uniform dispersion of fine alkaline-earth cuprate (AEC) and copper-free solid phases, whereas several low- J c conductors contain large AEC particles. Such significant differences in the phase morphologies in the melt are accompanied by a drastic difference in the formation kinetics of Bi-2212 during recrystallization cooling. STEM studies show that Bi-2212 grain colonies in the high- J c conductors have a high density of Bi 2 Sr 2 CuO y (Bi-2201) intergrowths, whereas a low- J c conductor, made using Bi 2.14 Sr 1.66 Ca 1.24 Cu 1.96 O x , is nearly free of them. STEM investigation shows grain boundaries in low- J c conductors are often insulated with a Bi-rich amorphous phase. High- J c conductors also show higher flux-pinning strength, which we ascribe to their higher Bi-2201 intergrowth density.